In the printed circuit board manufacturing process, there can be a significant interval between the production of the board substrate and the soldering of the various components to its surfaces. In fact, these two operations often take place at different sites. During the period of shipment and storage of the substrates, the external copper circuitry must be protected against oxidation in order to avoid problems with the subsequent soldering. The traditional way of doing this is to apply a coat of tin-lead solder to the copper, usually in a process known as hot air solder levelling (HASL). However, because of the environmental hazards associated with lead, and the fact that tin-lead coatings do not provide the flat, planar surfaces required for surface mount assembly, there is a rapidly growing trend towards the replacement of tin-lead by organic solderability preservative (OSP) coatings. Azole-type compounds have been found to be particularly effective for this purpose, and there exist numerous patents covering various derivatives of benzotriazole, imidazole, benzimidazole, etc. U.S. Pat. No. 3,933,531 to Sawa et al., U.S. Pat. No. 4,373,656 to Parker Jr. et al. and U.S. Pat. No. 5,173,130 to Kinoshita et al. describe such exemplary coatings. OSP coatings are usually applied from a weak acid solution (referred to hereinafter as an OSP solution) through formation of a complex between the azole and the copper surface. They are much thinner than tin-lead coatings. A typical thickness is 0.3 to 0.5 .mu.m, although some are applied at only 2 to 5 nm (see, I. Artaki et al, Circuit World, Vol. 19, No. 3, pp. 40-45, 1993).
Unlike tin-lead coatings, OSP coatings are invisible to the naked eye. This is a major disadvantage because it is impossible to know if a board has been treated, not treated or not completely treated, without resorting to destructive analysis (for example, dissolution of the coating and analysis by ultraviolet spectroscopy) or sophisticated instrumentation (for example, grazing angle infrared spectroscopy). One way of overcoming this problem is to colour the OSP coating when it is deposited. However, colour additives or dyes compatible with known OSP coatings are not readily available. To be effective, the dye must meet several criteria, namely:
(1) It must be soluble in the acidic solutions used to deposit the azole compounds. PA1 (2) It must be absorbed from the solution and deposited on the copper surface along with the azole. PA1 (3) During the subsequent soldering of surface components, it must not interfere with the soldering process and should preferably be removed by the solder flux. PA1 an azole compound suitable to provide a solderability protective layer on a metal surface, and PA1 a dye containing a cationic imine group having the general formula ##STR2## where X is an anion or an anionic group, the anion or anionic group selected such as to afford solubility of the dye in the azole compound, the deposition of the azole compound on the metal surface,.and solderability of the substrate.
It will certainly be appreciated that it is not practical to test thousands of various dyes, pigments and generally colouring substances to find one or more that meets the above criteria, without guidance or the benefit of hindsight.
It is an object of the present invention to provide compositions which are suitable as solderability preservative coatings for copper surfaces and are visible to the naked eye when applied on such surfaces.
It is another object of the invention to provide compositions meeting at least the above criteria (1-3).